e=i create graphical models of each cadaver shoulder, including the dummy sensors. An iterative closest-point algorithm [5] mapped a block to the surface of each sensor in the graphical models to determine the position of each sensor within the workstation coordinate system. The transformations describing the motion of each sensor during shoulder elevation were applied to the models to animate the shoulder kinematics (Fig. 2). Anatomic coordinate systems were defined for the scapula, clavicle, humerus, and spine of each model. For each bone, the iterative closes-point algorithm mapped geometric shapes to the bones to create the reference axes. The rotations of each bone were expressed as Euler rotations about the local coordinate axes, using the coordinate system fixed to the spine as a global reference system. Scapula and clavicle rotations were expressed as a function of humerus elevation.

The shoulder kinematics varied between elevation planes. For flexion and scapula plane abduction, all three of the scapula and clavicle rotations were less than 5° until the humerus and scapula became linked at approximately 90° of elevation. For coronal plane abduction, the bony and soft tissue constraints within the shoulder caused scapula and clavicle rotations on the order of 10° prior to 90° of elevation. The humerus and scapula became linked after 90° to 100° of elevation for all elevation planes. Once linked, the scapula rotations increased linearly with humerus elevation, and the clavicle rotations increased linearly with scapula lateral rotation. The results indicate that humerus elevation initiates passive shoulder motion and that bony and articular constraints within the joint vary between elevation planes.